(1) Field of the Invention
The present invention relates to an image coding apparatus and a method thereof, and particularly relates to an image coding apparatus and a method thereof for coding a picture which has been coded in a first coding scheme, in a second coding scheme.
(2) Description of the Related Art
In recent years, digital video recorders are increasingly popularized which digitalize streams of the Standard Definition Television (SDTV) such as the analog television broadcast, and compress-code them to be recorded on Hard Disk Drives (HDDs) and optical disks such as Digital Versatile Disk Random Access Memory (DVD-RAM) disks. Furthermore, digital video recorders have now begun to be commercialized which are capable of recording streams of the High Definition Television (HDTV) such as the satellite broadcast and the digital terrestrial high-definition television broadcast.
The streams transmitted in the HDTV broadcast are coded in the Moving Picture Experts Group (MPEG)-2 format. When the HDTV broadcast is recorded, it is recorded as the MPEG-2 streams. Alternatively, it is common to decode the images of the streams which have been coded in the MPEG-2 format; convert the decoded images into images having a size defined in the DVD standard; and code the resulting images in the MPEG-2 format before recording. Furthermore, there is also a method of coding, before recording, images of the streams which had been coded in the MPEG-2 format and then decoded, in the H.264 format with a higher coding efficiency compared to the MPEG-2 format. Coding in the H.264 format enables the recording of the HDTV images while suppressing image quality deterioration without changing the HDTV image size, even when the bit rate in the recording is lowered.
However, when coding in the H.264 format, the processing amount greatly increases since the H.264 standard, compared to the MPEG-2 standard, has a higher degree of freedom in motion compensation and there are more pictures which can be referred to.
Further, when coding high-definition (HD) images as in the HDTV broadcast, it is necessary to enlarge the search range in which motion estimation is performed, which in turn further increases the processing amount compared to the case of recording the SDTV broadcast.
The HDTV broadcast images are about six times larger in size than the SDTV broadcast images. Consequently, the recording of the HDTV broadcast even in the MPEG-2 format requires a search range having a size about six times larger than that in the case of recording the SDTV broadcast, in order to achieve about the same performance in the motion vector estimation as in the case of recording the SDTV broadcast.
In addition, compared to the case of recording (coding) the HDTV broadcast in the MPEG-2 format, recording (coding) the HDTV broadcast in the H.264 format requires a processing amount 36 times larger, since the number of macroblocks (MBs), the unit of coding, is six times greater.
Under the MPEG-2 standard, a maximum of two frames (four fields) can be referred to for motion compensation, whereas under the H.264 standard, a maximum of 16 pictures can be referred to. Therefore, the H.264 standard makes it possible to estimate more highly-accurate motion vectors than that in the MPEG-2 standard. However, under the H.264 standard, performing the motion vector estimation on every reference picture requires a processing amount eight times greater than that in the MPEG-2 standard. As described above, coding images in the H.264 format having a higher coding efficiency than the MPEG-2 format entails a problem that the processing amount becomes great compared to the case of coding pictures in the MPEG-2 format.
In view of such a problem, a coding method has been proposed for estimating accurate motion vectors even from narrowed search ranges (e.g. Patent Reference 1: Japanese Examined Patent Application Publication No. 11-308617).
The “digital image coder and motion vector detector used therefor” of the above mentioned Patent Reference 1 use, at the time of coding images, motion vector information of a previously coded picture for determining a search range for motion estimation.
However, in Patent Reference 1, using only the information of a previously coded picture to narrow the search range for the motion estimation makes it unable to follow the motion of the entire picture when an object in the picture suddenly moves in an opposite direction and/or moves at an accelerated speed, and thus, the accuracy of motion vector estimation deteriorates. Moreover, since the motion vector estimated by the accuracy-deteriorated motion estimation is used for determining a search range for further motion estimation, the search range is narrowed based on incorrect information, causing a further deterioration in the motion estimation accuracy. As a result, it is impossible to narrow the search range sufficiently.